Many processes and devices have been used for pressure sensing. Pressure sensors or pressure transducers are used in a wide range of applications. In many cases, it is desirable to measure the pressure of fluid media which may be harmful or corrosive to the transducer material, such as water, fuel, oil, acids, bases, solvents, other chemicals, and corrosive gases. There are numerous high-volume applications where a media compatible pressure transducer is highly desired but not available in any currently available technology with satisfactory durability, performance, or price characteristics. There is a need for media compatible pressure sensor packages which have substantial performance and cost advantages over existing technologies and provide new capabilities not previously realized.
Pressure is one of the most commonly measured physical variables. While pressure measuring instruments have been available for many decades, the proliferation of inexpensive solid-state silicon pressure transducers has resulted in tremendous growth in the number and different types of applications of pressure transducers. The most common pressure transducers are solid-state silicon pressure transducers employing a thin silicon diaphragm which is stressed in response to an applied pressure. The stress is measured by piezoresistive elements formed in the diaphragm.
Pressure transducers can also be formed similarly using metal foil diaphragms and thin film stress sensing elements. In some cases, one or two pressure sensing diaphragms are part of a parallel plate capacitor, in which the applied pressure is detected by the change in capacitance associated with the deflection of the loaded plate or plates. Other pressure measurement techniques include spring-loaded members which move in response to an applied pressure. For vacuum pressures there are a wide variety of other pressure measurement techniques.
Pressure transducers can be used to measure pressures in a wide variety of fluid media, including but not limited to: air, nitrogen, industrial process gases, water, automotive fluids, pneumatic fluids, coolants, and industrial chemicals. In many important applications, the media which the pressure transducer must measure is corrosive or damaging to the transducer itself. In these cases, the pressure transducer must either be constructed in such a way that it is resistant to the media of interest, or the transducer must somehow measure the pressure while being physically isolated from the media of interest. To date, pressure sensors are either inadequately protected for media compatibility or are prohibitively expensive for many applications.
Many different types of pressure sensors have been devised. The overwhelming majority of pressure transducers for media compatibility are protected by stainless steel housing, with a single stainless steel diaphragm providing a barrier between the pressure sensing element and the media. The empty volume between the steel diaphragm and the pressure sensing element is filled with a fluid, such as silicone oil. When the steel diaphragm deflects due to an externally applied pressure, the essentially incompressible fluid transmits that pressure to the internal pressure sensing element, which produces a voltage or current signal proportional to the pressure. While these stainless steel packaged pressure transducers are widely used, they have several shortcomings, including relative complexity and high cost.
In some industrial applications the rugged steel housing may be preferred regardless of price, there are numerous high-volume applications for media compatible pressure sensors in which the cost of the steel packages are prohibitively expensive. Also, the steel diaphragms, while thin, are inherently stiff due to the high modulus of steel. This results in a loss of sensitivity to applied pressure which is undesirable for transducer performance, especially at lower applied pressures.
These types of sensors are also inherently sensitive to temperature. A temperature rise causes the internal fluid to expand. Constrained by the steel diaphragm, the pressure of the fluid rises, producing a false pressure reading. This temperature sensitivity is typically corrected with external passive or active electronic components which add to the cost of the transducer. Fourth, the stainless steel material is not satisfactory for many media applications. Stainless steel will eventually corrode in certain environments with harsh acids and bases present. In some applications, such as in the semiconductor industry and biomedical applications, even if the steel is resistant to the chemical substance in question, minute trace amounts of steel or corrosion products released into the media cannot be tolerated. Also, steel housings add substantially to the weight and size of the transducers.
Examples of media isolated pressure transducers are known. One example involves a media compatible device for sensing pressure in which the media compatible pressure sensor utilizes self-aligned components which fit together without the need for adhesives is described. Other examples include relatively small-scale pressure sensing devices which are designed to be placed in close proximity to various substances from which pressure can be measured. In such devices the media compatible packages for pressure sensing devices can include molded polymeric housings and diaphragms which mount and isolate pressure sensing devices in operative contact with any type of corrosive or non-corrosive media for pressure measurement. The various pressure sensor package housings in such devices include a main cavity in which a pressure sensor is mounted, a polymeric diaphragm bonded to the housing within the main cavity, one or more media ports that leads to a pressure port on one side of the diaphragm, and a pressure transfer cavity on an opposite side of the diaphragm in which a pressure sensor is located.
Another example involves a media compatible package for a pressure sensing device is made from a non-corrosive, highly chemical resistant material. Such a package can include a base which holds a printed circuit board having a pressure sensor mounted thereon. A sealing member can be placed on the circuit board encircling the pressure sensor and a diaphragm can be disposed on the sealing member. In this example a fluid port can be attached to the base and compressingly engages the diaphragm creating a sealed chamber around the pressure sensor. The sealed chamber can be filled with a pressure transmissive fluid such as oil through a fill hole provided in the printed circuit board. This configuration allows the sensor package to be easily assembled with ordinary components and provides a design where all exposed surfaces of the package can be made from a highly corrosion resistive material.
Another media isolated pressure transducer involves deflecting a diaphragm differential pressure sensor is formed so all electrical elements and connections from external circuitry to the sensor are isolated from the pressure media. The deflecting, pressure sensing diaphragm is made of a semi-conductor material, having piezoresistors disposed on a surface thereof to form strain gages to sense deflection of the diaphragm. The strain gage resistors are media isolated by a layer that overlies the strain gage resistors.
Based on the foregoing, it can be appreciated that such prior art pressure sensor and transducer devices are plagued with a number of limitations. It is believed that a need exists for a low cost high accuracy media isolated pressure transducer that can be used in high temperature applications.